1. Field of the Invention
The present invention relates to a signal detecting circuit for a digital controller detecting a signal having a quantity corresponding to the state of the system (also referred to as a quantity of state) necessary for control from a signal detecting circuit in a digital controller such as an exciter for a synchronous machine or automatic voltage regulator, particularly from an instantaneous value of alternating current signal.
2. Description of the Related Art
A digital controller performs control by receiving an instantaneous value of alternating current from an instrument potential transformer or an instrument current transformer and detecting quantity of state necessary for control.
In the past, detection of the control signal has been performed totally by digital processing including removing a direct current component in the signal and a surge component at occurrence of an electric power system accident, as described in, for example, Japanese Patent Application Laid-Open No.63-305725 (1988). However, removing of the direct current component and the surge has been not sufficient. And another method is known in which the direct current component and the surge are absorbed by providing an analog filter as described in Japanese Patent Application Laid-Open No.4-213 (1992).
In the conventional methods described above, even when the signal has the direct current component and the surge at occurrence of an electric power system accident, the signal is detected in the same way as a normal state. Thereby, there is a disadvantage in that the control becomes instable. Therefore, it has been considered to solve this problem by providing an analog filter for removing the direct current component and absorbing the surge.